Devices and methods for length-wise exposure of photo conductors by a LED array are well known. A number of such devices have been disclosed including a device and method in which each individual LED in an array can be activated for a variable time, U.S. Pat. No. 4,074,320. Also see U.S. Pat. Nos. 3,482,039 and 4,040,094. However, of particular interest is British Application No. GB-A 2104266 which describes a printer in which a straight array of light-emitting diodes (LED's) is used to expose a photo-conductor in image-wise fashion. Since the radiated light intensities of the individual LED's, on energization, are not equal, a circuit is used in which the individual LED's are energized for a different time per image line to be imaged, so that each LED radiates substantially equal energy during an image line to be imaged. The energization time for each LED is thus controlled in inverse proportion to the light intensity of that LED. For this purpose, a 4-bit correction number is stored in a memory which is in inverse proportion to the relative energy delivery of this LED. An image signal is fed to this memory and multiplied by the corresponding 4-bit correction number. The resulting combined signal is then fed to a shift register of the serial-in-parallel-out type. Four outputs of the shift register are always combined, each via a tri-state buffer, and fed to a LED. The four switching inputs of the said buffers are each connected to an output of a signal generator. The signal generator delivers a time pulse per image line to each of the four switching inputs. The durations of these pulses are in the ratios of 8:4:2:1. Thus according to the contents of the connected shift register each LED is energized for a time which lies between the 0 and 15 time units, in such a way that the energy delivered per image line is the same for all the LED's. In order to control the light output of all the LED's simultaneously, the voltage across the array of LED's can be varied.
A disadvantage of the known method and device is that in the case of a moving photo-conductor the image points formed will not be on one line, because the energization times differ. Since in principle all the LED's are energized simultaneously, a very high switching current will flow through a long array of LED's, thus seriously interfering with other control electronics. Also, when grey tints are to be produced, obtained by generating small and large image points by means of the time control system described, the center of the small image points will be offset from the center of the large image points. The result is an unnatural imaging, which gives a disturbing effect particularly in the case of larger optical density transitions.
Another disadvantage of the known device is that when the light yield of all the LED's has to be changed by changing the voltage across those LED's, other correction values have to be applied, since the light yield as a function of the voltage applied differs for each individual LED.
It is, therefore, an object of the invention to provide a method and an exposure device which overcomes the disadvantages of the various prior art methods. It is a further object of the invention to provide a means of forming image points using LED arrays on a photoconductor.